gh-86986: Drop compatibility support for Sphinx 2 (GH-93737)

* Revert "bpo-42843: Keep Sphinx 1.8 and Sphinx 2 compatibility (GH-24282)"

This reverts commit 5c1f15b4b1024cbf0acc85832f0c623d1a4605fd

* Revert "bpo-42579: Make workaround for various versions of Sphinx more robust (GH-23662)"

This reverts commit b63a620014b67a6e63d10783149c41baaf59def8.
(cherry picked from commit 0efe3a1636c143fe0694a8e4d25d6eae19e0d618)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>